with a Launch Vehicle Data adapter (LVDA). The LVDA converted analog-to-digital and digital-to-analog with a Flight Control Computer (FCC). The FCC was an analog computer. Software architecture and algorithms LVDC instruction words were split into a 4-bit opcode field (least-significant bits) and a 9-bit operand address field (most-significant bits). This left it with sixteen possible opcode values when there were eighteen different instructions: consequently, three of the instructions used the same opcode value, and used two bits of the address value to determine which instruction was executed. Memory was broken into 256-word "sectors". 8 bits of the address specified a word within a sector, and the 9th bit selected between the software-selectable "current sector" or a global sector called "residual memory". The eighteen possible LVDC instructions were: Programs and algorithms In flight the LVDC ran a major computation loop every 2 seconds for vehicle guidance, and a minor loop 25 times a second for attitude control. The minor loop is triggered by a dedicated interrupt every 40 ms and takes 18 ms to run. Unlike the Apollo Guidance Computer software, the software which ran on the LVDC seems to have vanished. While the hardware would be fairly simple to emulate, the only remaining copies of the software are probably in the core memory of the Instrument Unit LVDCs of the remaining Saturn V rockets on display at NASA sites. Interrupts The LVDC could also respond to a number of interrupts triggered by external events. For a Saturn IB these interrupts were: For a Saturn V these interrupts were: Construction The LVDC was approximately wide, high, and deep and weighed . The chassis was made of magnesium-lithium alloy LA 141, chosen for its high stiffness, low weight, and good vibration damping characteristics. The chassis was divided into a 3 x 5 matrix of cells separated by walls through which coolant was circulated to remove the 138 watts of power dissipated by the computer. Slots in the cell walls held "pages" of electronics. The decision to cool the LVDC by circulating coolant through the walls of the computer was unique at the time and allowed the LVDC and LVDA (part-cooled using this technique) to be placed in one cold plate location due to the three dimensional packaging. The cold plates used to cool most equipment in the Instrument Unit were inefficient from a space view although versatile for the variety of equipment